Single inline packaged solid state relay with high current density capability

ABSTRACT

An AC solid state relay in a single inline package (SIP) comprised of dual silicon controlled rectifiers (SCRs) with a supporting circuitry mounted directly on an alumina substrate with molecularly bonded copper metalization layers and heat spreader all coated in a thermally conductive epoxy. And a DC solid state relay in a single inline package (SIP) comprised of an NPN power transistor with a supporting circuitry mounted directly on an alumina substrate with molecularly bonded copper metalization layers and heat spreader all coated in a thermally conductive epoxy.

This is a division of copending application Ser. No. 734,200, filed onJuly 22, 1991, now U.S. Pat. No. 5,134,094.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to solid state relays and morespecifically to small solid state relays, under twelve hundred watts,capable of switching AC and DC currents and of being constructed in asingle inline package (SIP).

2. Description of the Prior Art

As electromechanical devices, relays have filled an important role incontrolling large voltages and currents with relatively low controlpowers applied to the coils of the relays. Relays have also served toisolate the controlling circuit from the controlled circuit.

Advances in semiconductor technology have allowed the substitution ofelectromechanical relays with solid state relays. A practical limitationto how much power a solid state relay can switch has been the heatdissipation capability of the actual switching device and its package.Both silicon controlled rectifiers (SCRs) and triacs are used in solidstate relays and the load that can be placed on a solid state relay is afunction of the SCR/triac device rating and the thermal resistance ofdevice junction to the ambient air (for air cooling).

Schneider describes, in U.S. Pat. No. 4,172,272, issued Oct. 23, 1979, asolid state relay package that has a U-shaped metal frame. The open endof the frame receives a circuit board containing most of the relaycomponents and is potted in place with a solid plastic insulationmaterial. Schneider states that his package doubles the current ratingof a given relay circuit because his metal frame is so efficient atremoving heat. A triac assembly (31 in FIG. 9) is mounted to theinterior bottom of the metal frame by cementing or soldering. The use ofa triac in this fashion severely limits the kinds of loads that can beapplied to the relay, since an adequate dv/dt change in the controlvoltage must be maintained. Inductive loads, such as are found withmotors, are either curtailed or not allowed at all. The single triacdevice, as a switch point, tends to concentrate the heat the relay mustdissipate, and the efficiency with which heat is transferred from thejunction of the triac to the metal frame and then to the air limits theoverall power rating of the relay.

An earlier patent by Collins, U.S. Pat. No. 3,723,769, issued Mar. 27,1973, describes an optically isolated signal circuit for a solid staterelay. The relay also has zero-crossing switching with a full-wavebridge to reduce any radio interference that might otherwise begenerated. An AC load is controlled by two thyristors connected ininverse parallel such that each handles an opposite half cycle to theother. Collins suggests that a triac can be used to replace thethyristors, and yet does not address the dissipation of the heatgenerated in any of these devices, while Schneider is addressed tosubstantially only to the heat dissipation issue.

Copy machines and printers use high intensity halogen lamps that must beturned-on quickly. The initial surge current into one of these lamps canbe very high and exceed the capabilities of triac based solid staterelays. Halogen lamps have much higher inrush currents than doincandescent lamps. For a given area, a prior art solid state relayhaving a triac with an eighty amp surge capability will be unreliablebecause the maximum surge currents are being exceeded. With the presentinvention, the maximum surge current handling capability can be raisedin a device having the same area to 250 amp (using 180 mils² chips), andto 500 amp (using 240 mils² chips).

SUMMARY OF THE PRESENT INVENTION

It is therefore an object of the present invention to produce a solidstate relay having improved load capabilities for a given size package.

Briefly, in a preferred embodiment of the present invention, a solidstate relay comprised of dual silicon controlled rectifiers (SCRs) orone NPN bipolar transistor with the supporting circuitry is mounteddirectly on an alumina substrate with copper metalization and heatspreader and dipped in a thermally conductive epoxy.

An advantage of the present invention is that the surge capability of asolid state relay is improved.

Another advantage of the present invention is that higher currents canbe sustained by a solid state relay.

Another advantage of the present invention is that load restrictionsendemic to triacs are eliminated.

Another advantage of the present invention is that solid state relaydevice reliability is improved for a given package size and load.

These and other objects and advantages of the present invention will nodoubt become obvious to those of ordinary skill in the art after havingread the following detailed description of the preferred embodimentswhich are illustrated in the various drawing FIGS.

IN THE DRAWINGS

FIG. 1 is a schematic diagram of a first embodiment of an AC outputsolid state relay made in accordance with the present invention;

FIGS. 2A and 2B are a front and back views, respectively, of the relayof FIG. 1, showing the alumina substrate, SCR chips, heat spreader,input/output pins, and thick film components;

FIG. 3 is a schematic diagram of a second embodiment of a DC outputsolid state relay made in accordance with the present invention;

FIG. 4A and 4B are a front and back views, respectively, of the relay ofFIG. 3, showing the alumina substrate, NPN transistor, heat spreacer,input/output pins, and thick film components;

FIG. 5 is a typical derating curve for the solid state relay of FIG. 1;and

FIGS. 6A, 6B, and 6C are a front, bottom, and end view, respectively, ofa typical SIP package suitable for the solid state relay of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1, 2A, and 2B illustrate an AC solid state relay SIP assembly 10comprising an alumina ceramic substrate 11, having input/output pins 12,14, 16, and 18, and a heat spreader 20. Heat spreader 20 is preferablycopper metalization which is about 0.002 inches thick and is depositedonto substrate 11, as are the circuit traces on the opposite side ofsubstrate 11 that interconnect various components that form the solidstate relay circuit. Thick film resistor R1 is 470Ω, resistors R2 and R3are 47Ω, R4 and R5 are 150Ω, and R6 is 820Ω. Thick film resistors can besubstituted by discrete, leadless versions. PC1 is Siemens type IL420,or equivalent, integrated circuit for non-zero-crossing applications anda Toshiba TLP3063 or equivalent in zero-crossing alternativeembodiments. Silicon controlled rectifiers SCR1 and SCR2 are dice 180 or250 mils square and are conductively cemented or soldered tometallization pads on the front side of substrate 11 such that heat isdrawn from SCR1 and SCR2 into substrate 11 and spread by heat spreader20. The output power chips are covered by one of the commonly usedmaterials such as SILGAn J-500 or an equivalent for mechanicalprotection. Preferably, solid state relay 10 is then dipped in athermally conductive epoxy to completely seal substrate 11 and all thecomponents attached to it and may cover the tops of input/output pins12, 14, 16, and 18. An acceptable epoxy supplier is Hysol Corporation.The epoxy covering (not shown for clarity of the above description)protects the components of relay assembly 10 from moisture and abrasionand provides electrical insulation. The covering will ideally also addsome mechanical strength to the assembly and help spread heat generatedby SCR1 and SCR2 uniformly around relay assembly 10. In an exemplaryembodiment, substrate 11 is 96% alumina 0.025 thick, 0.85 inches wide,and 1.60 inches long. A dielectric glass layer using overglaze ink #8509(EMCA), or equivalent, may be applied. Input/output pins 12, 14, 16, and18 protrude 0.36 inches and are separated 0.20 inches to 0.49 inchesapart from one another. The following Tables I and II list the exemplaryinput and output specifications of solid state relay assembly 10 in thenon-zero-crossing alternative embodiment.

                  TABLE I                                                         ______________________________________                                        INPUT SPECIFICATIONS                                                          CHARACTERISTIC                                                                              UNIT    MIN      TYP  MAX                                       ______________________________________                                        V.sub.control Range                                                                         VDC     3         5   15                                        I.sub.control Range                                                                         mA      7        14   47                                        V.sub.pick-up VDC                    3                                        V.sub.drop-out                                                                              VDC     1                                                       V.sub.reverse VDC                   15                                        Protection                                                                    Input Resistance                                                                            Ohms             300                                            ______________________________________                                    

                  TABLE II                                                        ______________________________________                                        OUTPUT SPECIFICATIONS @  25° C.                                        CHARAC-   CONDI-                                                              TERISTIC  TION       UNIT     MIN   TYP  MAX                                  ______________________________________                                        V.sub.load                                                                              47-63 Hz   VRMS     12    120/ 280                                                                      240                                       V.sub.peak blocking                                                                     Repetitive VPK      600                                             I.sub.load (note 1)                                                                     Continuous ARMS     0.05       5                                    I.sub.surge (note 2)                                                                    one cycle  APK                 250                                  I.sub.leakage                                                                           Off-State  mARMS               1.0                                            @ 280 VRMS                                                          On-State  I.sub.load = MAX                                                                         VPK            1.1  1.6                                  Voltage Drop                                                                  Static dv/dt                                                                            Off-State  V/μS  500                                             Turn-On Time         μS               20                                   Turn-Off Time        mS                  8.3                                  I.sup.2 t Rating                                                                        t = 8.3 mS A.sup.2 S           260                                  ______________________________________                                         note 1: For temps. ≧ 30° C., see derating curve (FIG. 5).       note 2: Alternative 500A surge embodiment possible.                      

FIGS. 3, 4A, and 4B illustrate a DC solid state relay SIP assembly 110comprising an alumina ceramic substrate 111, having input/output pins112, 114, 116, and 118, and a heat spreader 120. Heat spreader 120 ispreferably copper metalization which is about 400 micro inches thick andis deposited onto substrate 111, as are the circuit traces on theopposite side of substrate 111 that interconnect various components thatform the solid state relay circuit. Thick film resistor R1 is 10KΩresistors R2 and R3 are 750Ω, resistor R4 is 62KΩ resistor R5 is 22KΩ,resistor R6 is 1KΩ, and resistor R7 is 680KΩ. Thick film resistors maybe substituted by discrete, leadless versions. PC1 is an opto-coupler.Transistor Q1 is a NPN in an industry standard semiconductor packagecommonly known as "SOT-23", transistor Q2 is a PNP in an industrystandard "SOT-89" package, and Q3 is industry standard "D-Pack" type NPNpower transistor cemented or soldered to metallization pads on the frontside of substrate 11 such that heat is drawn from Q3 into substrate 11and spread by heat spreader 120. Q3 may be in chip form. Preferably,solid state relay 110 is dipped in a thermally conductive epoxy tocompletely seal substrate 111 and all the components attached to it andmay cover the tops of input/output pins 112, 114, 116, and 118. Theepoxy covering (not shown for clarity of the above description) protectsthe components of relay assembly 110 from moisture and abrasion andprovides electrical insulation. The covering will ideally also add somemechanical strength to the assembly and help spread heat generatedmainly by Q3 uniformly around relay assembly 110. In an exemplaryembodiment, substrate 111 is 96% alumina 0.025 inches thick, 0.85 incheswide, and 1.60 inches long. A dielectric glass layer using overglaze ink#8509 (EMCA), or equivalent, may be applied. Input/output pins 112, 114,116, and 118 protrude 0.36 inches and are spaced 0.20 inches to 0.49inches apart from one another.

FIG. 5 is a derating curve for the solid state relay assembly 10,described above. It is estimated that the above solid state relays havea thermal resistance of 10° C. per watt. When the ambient airtemperature exceeds 30° C., the assembly 10 is less able to shed heatand the junction temperatures of SCR1 and SCR2 will rise to unacceptablelevels if the maximum current is not limited. For example, at an ambienttemperature of 60° C., the load current maximum will be just over threeamps. A similar derating curve to FIG. 5 will apply to solid state relay110.

FIGS. 6A, 6B, and 6C show one way the thermally conductive epoxy coatingcan be formed to cover solid state relay assembly 10. The epoxy forms ablock one inch by 1.70 inches by 0.50 inches. A mold can be used forthis purpose with the assembly 10 placed in the mold, molten epoxypoured in, and then allowed to cure according to the epoxymanufacturer's instructions. Certain relay applications will depend onthe solid state relay having a minimum thickness dimension. Instead ofthe square shape shown in FIGS. 6A-6C, it may be advantageous to coatthe solid state relay substrate and components with a thin epoxy coatingthat forms a skin-like membrane.

A major advantage in the use of copper for the metalization of thesubstrates 11 and 111 has been observed for alumina based substrates. Byapplying particular firing techniques and temperature profiles known inthe prior art of ceramic metallization, copper can be made tomolecularly bond with the alumina. If such a copper metalized aluminasubstrate is used in the above solid state relays, the life andreliability of the solid state relay will be substantially improved. Ata minimum, any peeling of the copper metalization layer over time, andafter running at near maximum operating temperature and currents, willbe reduced or eliminated.

Although the present invention has been described in terms of thepresently preferred embodiments, it is to be understood that thedisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artafter having read the above disclosure. Accordingly, it is intended thatthe appended claims be interpreted as covering all alterations andmodifications as fall within the true spirit and scope of the invention.

What is claimed is:
 1. A solid state relay single inline package (SIP)assembly, comprising:a plurality of input/output (I/O) pins aligned in asingle row for external connection; flat rectangular heat spreadingmeans for distributing heat uniformly throughout the SIP assembly andfor dissipation of said heat into an ambient air and for mechanicallysupporting the I/O pins; a first silicon controlled rectifier (SCR) indice form attached to the heat spreader means and for carrying apositive phase of an alternating current connected through the I/O pins;and a second silicon controlled rectifier (SCR) is dice form attached tothe heat spreader means separate from the first SCR and for carrying anegative phase of an alternating current connected through the I/O pinswherein heat produced by the first and second SCRs in carrying saidalternating current is divided in two and efficiently dissipated by theheat spreader means.
 2. The assembly of claim 1, wherein:the heatspreader means comprises a flat rectangular ceramic having coppermetallization substantially covering all of a first side of the ceramicand a plurality of copper metallization pads on a second side oppositeto said first side for attaching said first and second SCRs.
 3. Theassembly of claim 1, further comprising:epoxy coating means forprotecting the heat spreader means and first and second SCRs frommoisture and abrasion and for providing electrical insulation andmechanical strength.
 4. A direct current (DC) solid state relay,comprising:a flat alumina ceramic substrate including opposite first andsecond surfaces; a plurality of input/output (I/O) pins attached to thesubstrate along one edge and aligned in a single row for mechanicalsupport and electrical connection to a printed circuit assembly; heatspreader means deposited on said first surface of the substrate and fordistributing heat within the substrate uniformly throughout thesubstrate such that heat may be dissipated into an ambient environment;a plurality of metallization pad means on said second surface of thesubstrate; and a power transistor conductively attached to themetallization pad means and electrically connected to the I/O pins forconducting heat generated in the power transistor through themetallization pad means and distributed uniformly throughout thesubstrate by the heat spreader means.
 5. The relay of claim 4,wherein:the heat spreader means comprises a copper metallization layermolecularly bonded to the substrate.